Over the area in question the total thickness of the Chalk (where it can be measured from top to bottom, or nearly so) varies from 623 feet at Streatham to 1146 feet at Norwich, and in the latter case the topmost beds are absent; so that the full thickness, in some parts of Norfolk, may be 1200 feet or more. This figure, however, is exceptional, the nearest to it recorded being 890 at Harwich [this should probably be 895, the Glauconitic basal bed having been called Upper Greensand]. At East Horsley, in Surrey, a thickness of 817 feet has been proved, but generally speaking we find less, and over a large area 700 feet is not reached. It seems that the Chalk thins toward London (where 650 feet may be taken as about the usual thickness), not only from north and south but also from east and west. [It may be well to add the thicknesses proved by other borings outside London. These are as follows, where the borings have passed through the whole formation, from the overlying Tertiary beds to the underlying Upper Greensand or Gault:—Loughton 651 feet, Richmond 670, Cheshunt 681, Chatham and the neighbourhood 680–684, and Winkfield, near Windsor, 725. In sundry places where the topmost beds are missing we have the following:—Bushey (Herts) 686, Chartham (Kent) 735, and Coombs (Suffolk) 817, which last-mentioned should probably be over 820, for the same reason as given above, for increase at Harwich.]